Crohn's disease (CD) affects more than 600,000 individuals in the US and several million worldwide. Despite significant advances in our understanding of the mechanism(s) of chronic intestinal inflammation, the precise cause of the disease is still unknown. Therefore, available treatment modalities are not curative and possess significant side effects. Increasing evidence suggest that the interactions between diet and the gut microbiota may play a critical role in promoting or alleviating colitis. However, little is knon regarding the effects of diet composition and commensal flora on the pathogenesis of small intestinal inflammation, which is characteristic of CD. This renewal application will focus on the effects of dietary fats on the commensal flora in the pathogenesis of SAMP CD-like ileitis. The central hypothesis of this proposal is that the amount and type of dietary saturated fatty acids (SFAs) may have differential effects in regulating chronic intestinal inflammation by promoting either an anti-inflammatory or pro-inflammatory commensal flora. To test this hypothesis we will: 1) Characterize the effects of high-fat diets containing long chain (LC) and medium chain (MC) saturated fatty acids (SFAs) on the development of ileitis in SAMP mice. We will test the hypothesis that plant-based MC SFAs contained in coconut oil have a potent anti-inflammatory effect in SAMP CD-like ileitis, while plant and animal-based LC SFAs contained in cocoa butter and lard have proinflammatory effects. 2) Determine the impact of pro- and anti-inflammatory high-fat diets on the composition of gut flora and intestinal transcriptome. We will identify fundamental functional metatranscriptomic differences (i.e., microbial gene expression) between fecal and intestinal mucosa-associated floras in SAMP and AKR mice, and will precisely characterize commensal flora alterations (dysbiosis) resulting from the three high-fat diets using gut transcriptomics, 16S microbiome, qPCR and culture-based analysis to identify bacterial strains associated with pro or anti-inflammatory effects. 3) Determine the functional and immunological significance of commensal flora alterations induced by high-fat diet administration in germ-free (GF) SAMP mice. In a series of in vivo experiments, we will perform FMT studies from mice treated with the three high-fat diets into SAMP and AKR GF mice to precisely determine the functional and immunological effects of bacterial floral alterations induced by high-fat diets. This will include total FMT, as well as inoculation of specific bacteria strains. We will also determine whether the same effects observed with gut commensal mouse flora are also present with healthy and CD human fecal flora transplanted into SAMP and AKR GF mice. The overall objective of this proposal is to understand how different forms of dietary fat alter the commensal flora with the ultimately goal of either suppressing proinflammatory bacteria or enhancing the anti-inflammatory effects of commensal probiotics.